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The purpose of the present study is to examine the effects of different individual and group testing procedures on the success rate obtained by subjects on different formal problems. Four testing conditions were compared: a clinical interview; an individual interview without intervention; a group test with a demonstration of the material; and a group paper‐and‐pencil test. The four conditions are defined according to several variables that were deemed important for the explanation of potential performance outcomes. All subjects were administered the following four Piagetian derived tasks: a projection of shadows task; a permutation task; an equilibrium in the balance task; and an interrupter combinations task. According to the general hypothesis, subject performance should be optimal for the clinical interview and decline gradually as the assumed advantages of this method are systematically removed, i.e., counterquestioning of the examiner, retrials, subject‐examiner interactions, familiarization with the material, etc. The results show that there are performance differences between the conditions, but mainly across the individual and group situations. However, mean ranks obtained from each condition indicate that the facilitating variables identified in the first three conditions have a certain influence on subject performance. Implications regarding a choice of a valid mesurement of formal thought are discussed.